Attenuation equalizing circuit



C. W. GREEN ATTENUATION EQUALIZING CIRCUIT Filed April 24, 1926 IO fieyuency-fll/acyc/es June 24, 1930.

//1 vemor: Char/es W Green.

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Patented June 24, 1930 UNITED STATES PATENT oFFIcE 'CHARLES W. GREEN, OF MILLBURN, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK ATTENUATION EQUALIZIN G GIRCU IT Application filed April 24,

This invention relates to transmission systems employing repeaters for amplifying Waves of different frequencies simultaneously, and its principal object is to compensate for the unequal attenuation of such waves introduced elsewhere in the system.

The invention is particularly adapted for use in carrier current telephone and telegraph systems where several conversations or messages are transmitted over a single transmission line.

In systems of this type, due to the inherent attenuating properties of the transmission line and its associated apparatus, it is usually necessary to amplify the transmitted waves. This is usually accomplished by inserting repeaters at intervals along the transmission line, so arranged that all the waves transmitted in the same direction are amplified in acommon amplifier.

It is well known that in such systems, particularly those employing long transmission lines, the waves of higher'frequencies are at tenuated to a greater extent than the waves of lower frequencies. For this reason it is necessary not only to amplify all of the transmitted waves, but also to provide means for equalizing or compensating for the unequal attenuation of the waves of different frequencies in order that the repeaters may amplify all the waves to the same output level.

According to this invention the necessary equalization of the ;waves of different frequencies is obtained by providing a repeater circuit having a gain characteristic which is substantially complementary to the transmission loss characteristic of the transmission line and its associated apparatus. This is accomplished by neutralizing the capacities in the input circuit of the repeater, for example in the manner disclosed and claimed in my United States Patent No. 1,668,240, granted May 1, 1928, thus making the amplification of the repeater. elementsubstantially independent of the frequency that is to be ampli fied, and preferably making the impedance practically infinitely great at all frequencies to be amplified. An equalizing element is then introduced to compensate for the unequal attenuation by the line of the various 1926. Serial No. 104,362.

frequency components that are transmitted. In the usual case where the attenuationincreases at a fairly uniform rate with increasing frequency, this equalizing element'may take the form of an inductance, such as a transformer which serves to impress on the input of the repeater element a. voltage which increases with increasing frequency at a prop er rate. The rate of increase may conveniently be controlled by a resistance.

When this resistanceand the inductance in the input circuit of the repeater are proper 1y proportioned the gain characteristic of the circuit may be represented graphically by a substantially straight line showing a voltage amplification increasing from a minimum at the lowest frequency to a maximum at the highest frequency transmitted over the line. The slope of this characteristic curve may be varied by varying the value of the resistance in series with the primary winding of the input transformer to compensate for changes in the attenuation of the line due, for example,

In Fig. 1, a one-way repeater is employed to couple two line sections W and E. The signaling waves incoming at the repeater from line section W are amplified in the repeater and the amplified waves are delivered to line section E. A two-way repeater may be constructed by duplicating the circuit to provide a path for operation in the opposite direction.

The conductors of line section W are connected to the outer terminals of a. potentiometer comprising series resistance 5 and shunt resistances 6, 7 and 8. The resistances 6, 7 and 8 may be selectively connected to the primary windin of the input transformer 9 by means 0 an adjustable contact.

The amplifier 10 connected to the secondary winding of transformer 9 may be of the three-electrode space discharge type having a cathode, an anode and a control electrode. In the output circuit of the amplifier the Waves are transformed and reversed in phase in the transformer 11 from the secondary winding of which they are delivered to the line section E.

7 It is well known that the impedance between the input terminals of such a space discharge amplifier has the characteristic of a capacitive reactance which tends to resonate with the inductance of the input transs former, causing peaks in the transmission characteristic curve of the amplifier. The factors that enter into the input capacity of a space discharge amplifier include the interelectrode capacities, and the external capacities between the input terminals such as that of the Wirin and the shunt capacity of the input transformer windings. The in put capacities are shown dotted at 12 and 13, the capacity between the control electrode and the anode being indicated at 12, and the total direct capacity between the input terminals being indicated at 13.

In carrying out the invention these cap? cities are neutralized in order to produce, looking into the primary winding of the transformer 9, a substantially pure inductance the reactance of which is proportional to frequency. The input capacities may be neutralized, for example, in the manner disclosed and claimed in my copending application, mentioned above. As described in that application, this may be accomplished by providing a feed-back circuit in which an E. M. F. is generated substantially alike in phase to that impressed on the discharge tube input terminals and in which the magnitude of the current is controlled by an impedance of predetermined value. When the impedance of the feed-back circuit is adjusted to the proper value, the input impedance of the tube 10 measured across the secondary of transformer 9 becomes substantially infinite for a wide range of frequencies in so far as it afiects' the impressed waves, and its shunting efiect thereon'is reduced to a negligible amount. The E. M. F- fed back, which by reversal is brou ht into-like phase with the impressed E. M. is applied to the control electrode through a condenser 14: of predetermined lcapacity to furnish the current required by the input impedance of the amplifier.

It is evident that when the input capacities are neutralized, in the manner described above, the voltage across the cathode and control electrode of the amplifier 10'wil-l vary directly .1n response to changes in the fretransformer.

quency of the waves impressed upon the amplifier through the transformer 9, and the voltage amplification characteristic of the amplifier over the transmitted band of frequencies may be represented as a straight line. By inserting a resistance of proper value in series with transformer 9, the slope of this rising characteristic may be adjusted to any desired value.

The method of determining the desired amplification characteristic when the input capacities are neutralized in the manner described above will be considered by referring to Fig. 2.

In this circuit diagram R is the resistance in series with the open circuit voltage E at the repeater terminals. R is a resistance in series with the input transformenincluding the resistance of the primary winding of this The reactance of this'transformer is wL,tl'lcapacityeifect being neutralized as described above. The voltage across this reactance is the voltage which is impressed upon the amplifier. Designating this voltage 6, then If Rfl-R is large compared to 00L then, approximately,

Where m =21rf and 7, corresponds to the highest frequency which is to be equalized. Then,

where (.05 21rf w and f is assumed to correspond to the lowest frequency which is to be equalized.

The relation between 0 and c is then,

#4 m MIN .The variation in amplification, expressed the primary winding of 1 acteristic as T. 20log 1 hence under the conditions assumed, it will be seen by substitution that the total variation will be six transmission units. By using a potentiometer in conjunction with this circuit, as shown in Figs. 1 and 4, the gain of the amplifier may be varied. By giving the series resistances certain precomputed values, the slope of the amplifier charwell as its gain may be adjusted to meet the line requirements.

In Fig. 3 the curves a, b and 0 represent the gain characteristics of the amplifier for different values of R the abscissae representing the frequency in kilocycles and the ordinates representing gain in transmission units. Each of these characteristic curves is approximately a straight line over the frequency range extending from about 5,000 cycles to about 16,000 cycles which may represent, for example, the band of frequencies transmitted in one direction over the line section W. In each of'these cases the overall gain'of the repeater compensates'for the unequal attenuation of the waves of different frequencies incoming at the repeater. When the resistance 7 of Fig. 1 is inserted in series with the primary winding of transformer 9, for example, the overall gain characteristic of the repeater may be represented by the curve I) of Fig. 3. Similarly, the sloping gain characteristics represented by the'curves a and a may be obtained by connecting resistances 6 and 8, respectively, in series with the primary winding of transformer 9 to meet changing line requirements. Other resistances having different values may be employed to obtain different sloping characteristics.

The system of Fig. 4 comprises two space dischargetubes 15 and 16, the input and output circuits of which are connected together in well known push-pull arrangement. The

E. M. F. waves from the line section W are impressed upon the input terminals through the three-winding transformer 17, the ,two secondary windingsof which are accurately balanced to produce equal voltages. The output voltages of the two amplifiers being in opposition, each may be utilized as a source of E. M. F. for neutralizing the input impedance of the other in the manner described in my copending application, supra. Two feed-back paths are provided, each extending from the anode of one tube to the control electrode of the other and including a condenser 18. The resistances 6, 7 and 8 are adapted to be selectively connected in series with the primary winding of transformer 17 to give the repeater an equalizing action similar to that of the repeater described in connection with Fig- 1.

The invention has been described as applied to a repeater for use in multiplex carrier current signaling systems, but it is to be understood that it may also be applied to other places in the system than at repeater points, and to various types of systems other than multiplex carrier current'signaling systems.

What is claimed is: 1

1. In a wave transmission system employing waves of different frequencies, a line in which the waves of higher frequencies are attenuated to a greater extent than those of lower frequencies, a repeater including an appreciable inter-electrode capacity effect, a transformer for coupling the input circuit of said repeater to said line, means formaking the voltage across said input circuit substantially proportional to the frequency of the Waves impressed thereon comprising means for neutralizing said capacity, and means to control the rate of change of said voltage across said input circuit with frequency to a value such as to compensate for the unequal attenuation of the waves by said line comprising a resistance cooperating with said transformer.

2. In a wave transmission system employing Waves of different frequencies, a line in which the waves of higher frequencies are attenuated to a greater extent than those of lower frequencies, a repeater including an appreciable inter-electrode capacity effect, a transformer having a primary winding connected to said line and a secondary winding connected to the input circuit of said repeater, means for making the voltage across said input circuit substantially proportional to the frequency of the waves impressed thereon comprising means for neutralizing said capacity, and means to control the rate of variation of the voltage across said. input circuit with frequency to a value such as to compensate for the unequal attenuation of the waves by said line comprising a plurality of resistance elements adapted to be selectively connected in series with the primary winding of said transformer.

3. In a transmission system, a source of waves having a range of frequencies, a circuit for transmitting said waves, said circuit having higher attenuation at the higher frequencies of said range than at the lower frequencies, a repeater circuitconnected in said line, said repeater having appreciable inter-electrode capacity and having feedback means rendering the input impedance of said repeater substantially infinite throughout said range of frequencies insofar as the input impedance affects the impressed waves, common means for connectin said line to the input of said repeater am? compensating for the unequal attenuation of the comparison with pressing on an incoming difl'erent frequencies by said line, said common means including a shunt element whose reactance is substantially wholly inductive, a resistance in series with said line between said line and. said shunt inductance element, the value of said resistance being large in the reactance of said inductance element, and connections for imthe input of said repeater the voltage developed across said shunt element. 4. In a system for transmitting a range of frequencies at least as broad as the essential speech band, a line which attenuates the higher frequencies of the range to a greater degree than the lower frequencies, a resistance said line, a space discharge repeater having its input connected to receive the voltage developed in said inductance, and means for rendering the input impedance of said repeater substantially uniformly high throughout the range of transmitted frequencies comprising means for eliminating the regenerative amplification tendency of said repeater at all frequencies in the transmitted range, the amount of resistance-and of inductance used being determined with relation to the constants of the remainder of the system to reduce the excess attenuation of the higher frequencies by said line.

5. In a transmission'system for transmitting a wide range of frequencies, a line circuit which produces an excess attenuation of the higher frequency components over the lower frequency components of the range, a space discharge repeater possessing appreciable interelectrode capacity at frequencies in the transmitted range whereby said reand an inductance effectively connected to said waves transmitted thereover, comprising shunt inductance and series resistance, and a three-electrode discharge device amplifier having its input coupled to said inductance, said amplifier having means to render the amplification of the waves impressed upon it inductance substantially independent of the frequency Within the transmitted range including means to neutralize tfihe inter-electrode capacity of said amplier. In witness whereof, I hereunto subscribe my name this 15th day of April A. D., 1926.

CHARLES W. GREEN.

peater amplifies less eflectively at the high than at thelower frequencies, means comprising series resistance and shunt inductance to couple said repeater to said line, said means also serving to counteract the unequal attenuation of the different frequency components by said line and additional means to counteract the unequal amplifying effectiveness of said amplifier at the difierent frequencies. 7

6. A compensating repeater circuit comprising a repeater element possessing interelectrode capacity, means for making the impedance of the said repeater element substantially infinite at all frequencies within therange of frequencies to be repeated comprising means for neutralizing said interelectrode capacity, means making the voltage developed in the input of said repeater element substantially linearly proportional to the frequency throughout the transmitted range, comprising a shunt inductance element for coupling said repeater element to circuit, and a series resistance between the shunt element and the incoming' line-for adjusting :the rate of variation 

